Pathological gambling and drug addiction may share common neuronal substrates. Studies of cocaine- and alcohol-dependent individuals have revealed a higher prevalence of pathological gambling than seen in the general population often with gambling preceding drug addiction. Human imaging studies conducted during gambling and the performance of unpredictable reward tasks have also observed neuronal activation in the midbrain ventral tegmental area (VTA), a region intimately involved in the development and maintenance of addiction. Incentive sensitization provides a potential explanation for how this neuronal activation may underlie the transition from casual drug use to drug craving and abuse. In this model, repeated intermittent exposure to drugs, such as amphetamine, initiates neuroadaptations in the VTA that lead to sensitized locomotion and nucleus accumbens (NAcc) dopamine (DA) overflow in response to a drug challenge as well as enhanced amphetamine self-administration on a progressive ratio schedule. Sensitization has been observed following exposure to a number of drugs and cross-sensitization between drugs and non-drug reinforcers such as saccharin has been reported. Recently, we reported that exposure to conditions of gambling-like uncertain, rather than predictable, saccharin reinforcement may lead to the same neuroplastic changes in the brain that are produced by repeated intermittent psychostimulant exposure23. Non-deprived male rats were trained with escalating schedules of fixed-ratio (FR) or variable-ratio (VR) reinforcement for saccharin (FR: fixed relationship between presses and payout; VR: uncertain relationship between presses and payout). At the end of training, both groups worked reliably on the schedules and total saccharin intake did not differ between groups. Nonetheless, a threshold amphetamine challenge injection (0.5 mg/kg IP) administered two weeks after the last saccharin training session produced a significantly greater locomotor response in VR compared to FR rats. This finding suggests that the variable reinforcement schedule elicited neuroplastic changes similar to those produced by repeated psychostimulant exposure known to sensitize responding. In two aims, the proposed experiments will begin to characterize how repeated exposure to gambling-like unpredictable reinforcement promotes locomotor cross-sensitization to amphetamine and determine whether amphetamine self-administration and NAcc DA overflow are also subsequently enhanced. First, midbrain DA neuron reactivity will be assessed during responding for uncertain reward by measuring somatodendritic DA overflow in the VTA while rats respond for predictable or unpredictable saccharin reinforcement. Second, the long- lasting effects of exposure to uncertain reinforcement will be further characterized by testing whether this experience leads to both enhanced drug self-administration and NAcc DA overflow. These experiments have important implications for understanding the neuroadaptations that underlie different forms of addiction and are the first to investigate this potential transition from pathological gambling to drug addiction in an animal model.